1. Field of the Invention
This invention relates to an improvement in the process for the production of ethylene oxide through the silver catalyzed vapor phase oxidation of ethylene and, more particularly, to an economic process for producing ethylene oxide which minimizes unreacted ethylene losses through the use of semipermeable membrane separation units, thereby permitting an effective, selective removal of argon diluents from the process cycle gas, without significant ethylene losses.
2. Description of the Prior Art
The production of ethylene oxide is one of the most important commercial reactions in the world, with a current annual production of about eight million tons/year worldwide. Frequently the operation of a process of this magnitude within the realm of commercial practicality can often depend upon the ability to increase, even by only relatively small amounts, the ethylene oxide yield or, conversely, the ability to reduce, even by only relatively small amounts, the costs of a variety of essential elements of the process.
In one major embodiment of the process, the oxygen supplied for reacting with ethylene is obtained from an expensive oxygen source providing essentially relatively pure oxygen, i.e., 95+%, along with a substantially lesser but yet significant fraction of one or more diluents, e.g., nitrogen, argon and the like. When added to the process cycle gas, usually in conjunction with a makeup ethylene stream, the oxygen and ethylene mix with a controlled amount of other diluents, other reaction products and contaminants, i.e., carbon dioxide, argon, nitrogen, methane, etc. The precise concentration of the ingredients which can be included is a significant consideration in determining the composition of the reaction gas since the prior art has had no economic way of selectively purging some of these impurities, particularly nitrogen and argon, without also losing large amounts of valuable feed gases, i.e., ethylene. In commercial scale plants excess CO.sub.2 is usually chemically absorbed from the reaction recycle gas by contacting the gas stream with a hot potassium carbonate solution, stripping out the CO.sub.2 with steam, and venting it into the atmosphere with a minimal loss of ethylene. Although such absorption units are expensive and also require significant amounts of energy during operation, they are still economically justified by the reduction in ethylene losses which they provide. In the past the art has controlled the concentration level of diluents such as argon and nitrogen by a less expensive purge type system, i.e., a venting or bleeding step, which unfortunately also results in the loss of a substantial amount of ethylene and therefore a reduced ethylene oxide yield.
A significant consideration in forming a suitable reaction gas composition, which typically is made of a majority of diluent ingredients, along with O.sub.2 and C.sub.2 H.sub.4, is to form a composition which avoids very high and unstable concentrations of oxygen and/or ethylene. Otherwise, a total combustion of the ethylene content, forming CO.sub.2 and water as the reaction products, as well as the danger of an explosion caused by exceeding the flammability limit of the gas composition, can take place. The amount of argon present in the gas is particularly important regarding the flammability issue, since argon is a particularly ineffective component for reducing the flammability of the gas mixture. Thus, a method of selectively monitoring and economically removing certain components, particularly argon, without a substantial loss in reactant gas, particularly ethylene, has been a long felt need in the art.
U.S. Pat. No. 3,083,213 is an early teaching on the undesirability of the presence of argon, particularly regarding the flammability factor, in a disclosed ethylene oxide process. U.S. Pat. No. 3,119,837 discloses that methane can be a suitable diluent under certain circumstances in an ethylene oxide process. British Pat. No. 1,321,095 discloses an ethylene oxide process wherein ethylene levels as high as 40-80% by volume are permitted, and in which several diluents are controlled through means of a vent mechanism.
In copending case Ser. No. 729,340 of Brian Ozero, filed concurrently with the present application, the usage of semipermeable membranes to selectively remove CO.sub.2 and argon from an ethylene oxide recycle stream is disclosed; the process of Ser. No. 729,340 employs a low purity oxygen feed stream and preferably does not utilize another unit operation to remove CO.sub.2 from the system.
It is an object of this invention to provide a process for the selective removal of argon from the ethylene-oxygen recycle gas mixture.
It is another object of this invention to provide a process for the removal of argon, and accompanying maintenance of the process cycle gas at desired argon concentrations, without an accompanying substantial loss of ethylene, from the cycle gas.